As having good mechanical properties, good moldability and good electric insulating properties, thermoplastic resins are heretofore used in various fields, for example, for parts of electric and electronic appliances for household use, OA appliances, automobiles, etc. In some fields of their applications, however, thermoplastic resins are required to be resistant to flames for safety, and various techniques have been proposed for making thermoplastic resin products resistant to flames.
For this, generally employed is a method of adding a halogen-containing flame retardant of, for example, bromine compounds having a high flame-retarding ability to thermoplastic resins, along with antimony oxide thereto, so as to make the resins resistant to flames. However, the method is problematic in that the resulting resin compositions much smoke when fired.
To solve the problem with such halogen-containing flame retardants, therefore, the recent strong demand in the art is for flame-retardant resins not containing a halogen at all.
Phosphorus-containing flame retardants are known for non-halogen flame retardants, and their typical examples heretofore used are phosphates. For example, disclosed are a method of adding a polyphosphate to a thermoplastic resin (Japanese Unexamined Patent Application Publication No. 24736/1984); a method of adding a phosphate having a specific structure to a rubber-reinforced styrene (Japanese Unexamined Patent Application Publication No. 140270/1999); and a method of adding a liquid phosphate to a styrene resin (Japanese Unexamined Patent Application Publication No. 5869/1999).
However, for naturally combustible thermoplastic resins such as typically styrene resins, the flame-retarding effect of phosphates is extremely poor. Therefore, in the thermoplastic resin compositions obtained in the methods of Japanese Unexamined Patent Application Publication Nos. 24736/1984, 140270/1999 and 5869/1999, the amount of the phosphate must be large for making the resin compositions resistant to flames. As a result, the mechanical properties of the resin compositions are worsened, and, in addition, the phosphate often bleeds out of the resin moldings. Other problems with them are that the resin compositions soil the molds used for molding them and that they produce gas while molded.
To solve the problems as above, a method of using a hydroxyl group-containing phosphate is disclosed in Japanese Unexamined Patent Application Publication No. 247315/1993.
However, the hydroxyl group-containing phosphate ester could not solve the above-mentioned problems, as its flame-retarding effect is still extremely poor.
Having experienced the poor flame-retarding effect of phosphates, we, the present inventors tried combining a phosphate with a salt of melamine-cyanuric acid that serves as a flame retardation promoter for enhancing the flame-retarding effect of the phosphate, and found that the combination was effective for increasing the flame retardancy of thermoplastic resins containing it. However, we could not still solve the problem with the resin compositions in that their mechanical characteristics, impact resistance and moldability intrinsic to the thermoplastic resin therein are after all worsened by the combined additives added thereto.
On the other hand, a method of adding a novolak-phenol resin that serves as a carbonized layer-forming polymer, and also a triazine skeleton-containing compound to a hydroxyl group-containing phosphate for enhancing the flame-retarding effect of the phosphate is disclosed in Japanese Unexamined Patent Application Publication No. 70448/1995.
However, the method could not still solve the problem with thermoplastic resin compositions in that their mechanical characteristics, impact resistance and moldability intrinsic to the thermoplastic resin therein are after all worsened by the combined additives added thereto. In addition, since the phenolic resin used in the method is not resistant to light, the resin composition obtained by the method is further problematic in that its lightfastness is poor.
The present invention is to solve the above-mentioned problems, and its object is to provide a flame-resistant thermoplastic resin composition of which the flame retardancy is highly improved and which has good mechanical characteristics and good heat resistance, especially good lightfastness.